I assume that, since normally when a relationship is created a "symmetric" relationship is created from the child to the parent. Asymmetric therefore would mean that only the ParentChild-Relationship is created.
Thanks a lot for the information.
This statement is listed in the 2016.2.1 help:
Specifies if the relationship is asymmetrical, which means that there is no reverse relationship.
The system automatically populates this field and you cannot change it.
This doesn't help much. But a case came up where I wanted to use something like this. But it appears you can't set it.